1. Field of the Invention
The present invention relates to a photovoltaic powered system, and more particularly to a grid-connected photovoltaic powered system.
2. Description of Prior Art
Because the solar energy has the pollution-free and public harm-free characteristics and is further inexhaustible in supply and always available for use, the solar energy has high potential applications and developments. Recently with the rapidly development of the high-efficiency solar cells, this topic has been gradually promoted by making policies in many developed countries, such as Europe countries, the United States, Japan, and so on.
The solar photovoltaic system provides a photovoltaic conversion to generate a DC power through the solar cell panels. Afterward, the DC power is converted into an AC power through a power conditioner to supply to loads or the converted AC power is grid-connected to an AC utility power through the utility grid bus. The solar photovoltaic system can be broadly divided into three categories: (1) stand-alone system, (2) grid-connection system, and (3) hybrid system.
The stand-alone system means that the solar photovoltaic system is completely operational without requiring external support and only directly supply to loads. Hence, the stand-alone system is generally built in remote areas or isolated islands. In particular, the required power electricity of loads is either the wind power or the solar power. The solar power or/and the wind power can further provide redundant power to charge the standby battery, whereas the loads can be supplied through the battery when the solar power or/and the wind power is insufficient. The grid-connection system means that the solar photovoltaic system is further connected to the power grid of the electric power company. Hence, the grid-connection system is suitable for where the utility power can reach. When the amount of electricity generation of the solar photovoltaic system is greater than that of load demands, the redundant power remains would be delivered to the utility grid bus. On the other hand, the utility power can provide the required power electricity to loads when the amount of electricity generation of the solar photovoltaic system is insufficient. Furthermore, in order to improve the power supply reliability and quality, the hybrid system is developed. The solar photovoltaic system, which is combined with standby batteries, is temporarily separated from the utility power to provide power electricity to loads when the utility power fails. The solar photovoltaic system is further grid-connected to the utility grid bus until the utility power is available.
The grid-connected solar photovoltaic system can be divided into two categories: single-stage structure and two-stage structure. Reference is made to FIG. 1A and FIG. 1B which are a block diagram of a prior art single-stage and a prior art two-stage grid-connected photovoltaic system, respectively. As shown in FIG. 1A, the photovoltaic system is grid connected to an AC utility power 50A. The photovoltaic system includes a photovoltaic module 10A, an input filter capacitor 20A, and a DC/AC converter 40A. The photovoltaic module 10A converts light energy into electric energy to provide a DC output voltage Vpv and a DC output current Ipv. The input filter capacitor 20A is electrically connected to the photovoltaic module 10A to provide functions of energy-storing, energy-releasing, and filtering for rear-end circuits. The DC/AC converter 40A is electrically connected to the input filter capacitor 20A, and the power switches of the DC/AC converter 40A can be driven through a sinusoidal pulse-width-modulation (SPWM) technology, thus converting the DC power into the amplitude-modulated and frequency-modulated sinusoidal AC power. Hence, the modulated sinusoidal AC power can be grid-connected to the AC utility power 50A through the utility grid bus. Because the single-stage grid-connected photovoltaic system has the advantages, such as simple circuit structure, easy control, and high efficiency, it is suitable for the large power photovoltaic module with a high output voltage. Because the photovoltaic module 10A is directly connected to the high-voltage DC bus of the DC/AC converter 40A, the single-stage grid-connected photovoltaic system is not suitable for the small power photovoltaic module due to the inelastic arrays of the photovoltaic module 10A and low-reliable in-series batteries.
As shown in FIG. 1B, a major difference between the single-stage structure and the two-stage structure is that the two-stage grid-connected photovoltaic system further has a DC/DC converter 30A. The DC/DC converter 30A is electrically connected to the input filter capacitor 20A to receive the filtered DC voltage outputted from the input filter capacitor 20A and boost the voltage level of the filtered DC voltage. The operation of the rear-end circuits of the two-stage structure is identical to that of the single-stage structure, hence, the detail (description) is omitted here for conciseness. Because many in-series photovoltaic modules 10A are not required in the two-stage grid-connected photovoltaic system, it could reduce volume and costs and increase elasticity and reliability of the array connection of the photovoltaic module 10A. However, the two-stage structure and the high-frequency switching would reduce efficiency of the photovoltaic system.
The above-mentioned conventional signal-stage or two-stage grid-connected photovoltaic system generally uses the DC/AC converter 40A with a high-frequency switching operation. In particular, the output amplitude, frequency, and phase of the photovoltaic system have to the same with those of the AC utility power 50A. In addition, a phase-locked-loop (PLL) is used to provide a synchronous operation for the DC/AC converter 40A to connect to the AC utility power 50A. Furthermore, a good current-controlling operation is necessary to prevent damaging the converters from circulating current when the photovoltaic system grid-connecting to the AC utility power 50A. In addition, the major disadvantages of the DC/AC converter 40A are the poor self current-limiting and poor current-controlling, thus easily occurring transient over-current damage.
Accordingly, it is desirable to provide a photovoltaic powered system to overcome disadvantages of the DC/AC converter 40A and increase whole conversion efficiency, thus increasing the utilization rate of the photovoltaic powered system.